The present invention concerns retractable stop mechanisms and more particularly such mechanisms for preventing unauthorized removal of covers for catch basins and manholes installed in roadways for access to utilities, i.e., water, gas, communications, electricity, etc. These covers are provided in order to keep out debris but also to present a supporting surface for pedestrians and vehicles on the surrounding roadway surface. If the covers are removed by thieves or vandals, the opening to a catch basin or manhole presents severe hazard to vehicles and pedestrian traffic. Removal of these covers by vandals or scavengers has become an increasing problem which has created an urgent need for some means for preventing unauthorized removal of the covers. In instances where there are no existing means incorporated in existing catch basin and manhole covers, providing a simple retrofit to existing covers with a securing means is highly desirable. Such devices should preferably be designed to withstand traffic and weather and the efforts of vandals, thieves or terrorists to remove them. The large numbers involved require a simple installation method to minimize labor costs. The securing means should preferably not present an obstruction in the roadway, particularly for snow plows, nor a tripping hazard to pedestrians.
At the same time, any such means should be reliable in operation even after many years of exposure to the elements.
The present inventor has heretofore invented a retractable stop mechanism which is designed to allow a limited movement of a manhole cover out of the manhole vault structure to when an explosion occurs in the manhole space but prevents the cover from being blown free so as to avoid the hazard presented by the manhole cover when it falls back to the surface while still allowing escape of the gases so as to minimize damage to the vault structure. The stop mechanism also prevents complete removal of the manhole cover to function as a security retention for the cover. This is described in detail in U.S. Pat. No. 7,484,908 incorporated herein by reference. That mechanism relies on a spring return to retract the stop member, but due to the possibility of corrosion, causing increased resistance to movement of the parts of the mechanism, it is desirable to not rely on a spring operation of the retraction release as the mechanism since it may be exposed to the elements for many years.
It is an object of the present invention to provide a simple but selectively and reliably releasable stop mechanism for manhole covers and other structures which can resist attempts at unauthorized removal of the cover, but which is easily operated by an authorized person even after many years in service, which can be readily installed on existing covers, and does not present an obstruction to traffic.
It is a further object to provide a releasable cover stop mechanism advantageously arranged to allow a manhole cover to rise up out of a manhole cover frame to a limited extent for venting gases created in an explosion as described and claimed in U.S. Pat. No. 7,484,908.